The layers of the atmosphere appear as different colors in this image from the International Space Station.

Air Temperature

The atmosphere is layered, corresponding with how the atmosphere’s temperature changes with altitude. By understanding the way temperature changes with altitude, we can learn a lot about how the atmosphere works.

Papers held up by rising air currents above a radiator demonstrate the important principle that warm air rises.

Warm Air Rises

Why does warm air rise (
Figure
above
)? Gas molecules are able to move freely, and if they are uncontained, as they are in the atmosphere, they can take up more or less space.

When gas molecules are cool, they are sluggish and do not take up as much space. With the same number of molecules in less space, both air density and air pressure are higher.

When gas molecules are warm, they move vigorously and take up more space. Air density and air pressure are lower.

Warmer, lighter air is more buoyant than the cooler air above it, so it rises. The cooler air then sinks down, because it is denser than the air beneath it. This is convection, which was described in the chapter Plate Tectonics.

Temperature Gradient

The property that changes most strikingly with altitude is air temperature. Unlike the change in pressure and density, which decrease with altitude, changes in air temperature are not regular. A change in temperature with distance is called a
temperature gradient
.

Layers

The atmosphere is divided into layers based on how the temperature in that layer changes with altitude, the layer’s temperature gradient (
Figure
below
). The temperature gradient of each layer is different. In some layers, temperature increases with altitude and in others it decreases. The temperature gradient in each layer is determined by the heat source of the layer (See opening image).

The four main layers of the atmosphere have different temperature gradients, creating the thermal structure of the atmosphere.

Most of the important processes of the atmosphere take place in the lowest two layers: the troposphere and the stratosphere.